Outline

Background: Proton therapy is one form of radiotherapy in which the irradiation can be concentrated on a tumor using a scanned or modulated Bragg peak. Therefore, it is very important to evaluate the proton-irradiated volume accurately. The proton-irradiated volume can be confirmed by the detection of pair annihilation gamma rays from a positron emitter nuclei generated by the nuclear fragmentation reaction between the incident proton and nuclei in the human body. This is accomplished by using a beam ON-LINE PET system (BOLPs) which uses PET images to carry out dose-volume delivery guided proton therapy (DGPT). The purpose of our study is to verify the utility of this developed system in clinical use.

Material and method: In the proton treatment room, the BOLPs mounted on the rotating gantry port (BOLPs-RGp) were constructed so that a planar PET apparatus with spatial resolution as high as 1.6–2.1 mm could be mounted with the field of view covering iso-center of the beam irradiation system. The useful field size for the detection area is 164.8Ã167.0 mm2. The BOLPs-RGp is set up with a 300-900 mm distance between the upper and lower opposing detector heads. The detector heads rotate around the iso-center synchronous with the rotating gantry port. Activity measurements were performed in 100 patients with tumors of head and neck, liver, lungs, prostate, and brain. The position and intensity of activity were measured using the BOLPs-RGp immediately after the proton irradiation. The measurement time was carried out between a period of 210–500 seconds (beam off: 200 seconds = constant) from the start of the proton irradiation. The measured data were stored in a computer for image display and for the analysis of the binary formatted data pixels with 1.1-mm size mapped on a frame of 164.8Ã167.0 mm2 every second.

Results: The daily measured activity-images acquired by the BOLPs-RGp showed the proton irradiation volume in each patient. Changes in the proton-irradiated volume were indicated by differences between a reference activity-image (taken at the first treatment) and the daily activity-images. In the case of head and neck treatment, the activity distribution changed in the areas where partial tumor reduction was observed. In the case of liver treatment, it was observed that the washout effect in necrotic tumor cells was slower than in non necrotic tumor cells.

Conclusion: The BOLPs-RGp was developed for the DGPT. The accuracy of proton treatment was evaluated by measuring changes of daily measured activity. Information about the positron-emitting nuclei generated during proton irradiation can be used as a basis for ensuring the high accuracy of irradiation in proton treatment. Figure 1 [Fig.Â 1], Figure 2 [Fig.Â 2].